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    <title>物理层</title>
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<h2>基本概念</h2>

<h2>数据通信基础</h2>

<h3>通信频谱</h3>

<table>
	<tr>
		<th>英文缩写</th>
		<th>英文</th>
		<th>中文</th>
		<th>频率</th>
		<th>波长</th>
		<th>波的类型</th>
	</tr>
	<tr>
		<td>LF</td>
		<td>Low Frequency</td>
		<td>低频</td>
		<td>30-300kHZ</td>
		<td>1-10km</td>
	</tr>
	<tr>
		<td>MF</td>
		<td>Medium Frequency</td>
		<td>中频</td>
		<td>300kHZ-3MHZ</td>
		<td>100m-1km</td>
	</tr>
	<tr>
		<td>HF</td>
		<td>High Frequency</td>
		<td>高频</td>
		<td>3-30MHZ</td>
		<td>10m-100m</td>
		<td>短波</td>
	</tr>
	<tr>
		<td>VHF</td>
		<td>Very ...</td>
		<td>甚高频</td>
		<td>30-300MHZ</td>
		<td>1m-10m</td>
	</tr>
	<tr>
		<td>UHF</td>
		<td>Ultra ...</td>
		<td>特高频</td>
		<td>300MHZ-3GHZ</td>
		<td>10cm-1m</td>
		<td>微波</td>
	</tr>
	<tr>
		<td>SHF</td>
		<td>Super ...</td>
		<td>超高频</td>
		<td>3-30GHZ</td>
		<td>1-10cm</td>
	</tr>
	<tr>
		<td>EHF</td>
		<td>Extremely ...</td>
		<td>极高频</td>
		<td>30-300GHZ</td>
		<td>1mm-1cm</td>
	</tr>
	<tr>
		<td>THF</td>
		<td>Tremendously ...</td>
		<td></td>
		<td></td>
		<td>0.1-1mm</td>
	</tr>
</table>

<p>	注：频率 <math><mi>f</mi></math> 与波长
	<math><mi>&lambda;</mi></math> 相乘等于光速:
	<math><mi>c</mi><mo>=</mo><mi>&lambda;</mi><mi>f</mi></math>.
</p>

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